SCR catalysts remove nitrogen oxides (NOx), often the most abundant and polluting component in exhaust gases, through a chemical reaction between the exhaust gases, a reducing agent, and a catalyst
Urea-based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea, also known as carbamide ((NH2)2CO), is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into gaseous ammonia (NH3) and is stored in the catalyst. The NOx contained in the engine exhaust gas entering the catalyst then reacts with the stored ammonia, which produces nitrogen and water. The amount of urea (reductant precursor) injected is controlled to provide the maximum NOx conversion efficiency whilst keeping excess NH3, also known as NH3 slip, to low values.
SCR catalysts have mainly been introduced on heavy-duty vehicles where high NOx levels are present and where steady state can be considered to be the main operating conditions. In these conditions, SCR control consists of supplying a certain NH3 to NOx ratio, usually mapped as a function of speed and load.
Applying this kind of control on a passenger car, where transient conditions are more frequent, usually requires specific transient corrections. Moreover, vanadium based catalysts are often used on heavy duty vehicles and this technology is known to have a reduced buffering effect (the temporary storage of NH3) than new Zeolite based catalysts (Fe, Cu) used on passenger car (or light duty) applications. In this connection, it is now known that the NOx conversion efficiency depends on the amount of ammonia stored therein, while it is not necessary that all of the catalyst storage capacity be utilized by ammonia to achieve the optimal NOx conversion efficiency. Additionally, the NOx conversion efficiency also depends on the catalyst temperature.
As a result, SCR control generally involves:                an open-loop dosing of reducing agent based on maps or based on chemical modelling of the SCR system;        closed-loop correction of the dosing with a post-SCR exhaust sensor, typically a NOx sensor.        
A difficulty of closed-loop control of such a system is that NOx sensors are sensitive to NH3. In practice, this implies that when excess urea is injected and converted to NH3, the post-SCR NOx sensor detects the NH3 breakthrough (slip) as an increase in NOx, which is thus interpreted by conventional controllers as a need to increase the reducing agent dosing, while the urea injection should actually be reduced.
This cross-sensitivity to NH3 thus creates additional difficulty that is required to be taken into account in the control of SCR catalyst systems in order to avoid wrong dosing actions.
Hence, there is a need for an improved method for the closed-loop control of a SCR catalyst. This is achieved by a method for controlling a SCR catalyst as claimed in claim 1.